Competitive Interactions Among Crustose Coralline Algae

By: Gavin W. Maneveldt & Derek W. Keats

Botany Department, Univ. of the Western Cape, P.Bag X17, Bellville 7535 South Africa

In intertidal communities, much of the primary substratum is occupied by encrusting coralline algae (Adey & McIntyre, 1973; Johansen, 1981; Paine, 1984; Steneck, 1986; Steneck & Paine, 1986; Dethier et al., 1991; Steneck et al., 1991; Keats & Maneveldt, 1994; Keats et. al., 1994). Their essentially two dimensional mode of growth, via meristematic cells located near the margins of the thallus, means that they often come into direct contact with other benthic organisms and undergo interference competition for space (Steneck, 1986; Steneck et al., 1991; Keats & Maneveldt, 1994). It is thought that encrusting algae compete for space mainly by overgrowing one-another where leading margins come into contact (Steneck, 1985; 1986; Steneck et al., 1991).

Based on ours and other studies (Steneck, 1985; 1986; Steneck et al., 1991), we can recognise four outcomes of competition involving encrusting algae. Thicker crusts generally overgrow thinner ones (Fig. 1). Crusts of equal thickness frequently reach a competitive stand-off, in which neither species wins at overgrowth competition (Fig. 2). Crusts with raised margins will overgrow crusts with adherent margins as long as the degree to which the margin is raised, is greater than the thickness of the adherent crust (Fig. 3). In species which spontaneously regenerate secondary margins at their surfaces (as shown by Leptophytum foveatum [Keats & Maneveldt, 1994]), the thinner crust may slow or stop the overgrowth by a thicker crust by regenerating a secondary margin which may grow up and even grow over the margin of the encroaching alga (Fig. 4). Further studies are however needed in other habitats and different geographical locations to determine the broader applicability of this model.

(The following figures depict the types of overgrowth competition among crustose algae. The green crust is crust A, while the red crust is crust B. The ">" symbol means "overgrows" ,so that A>B means crust A overgrows crust B. The "=" symbol means "reaches a competitive stand-off", while the "><"symbol means the first crust overgrows the second one, but the second crust "retaliates" to overgrowth by the first crust. Click the figure to see actual examples of these overgrowth interactions)

Figure 1. Thick crusts have a competitive advantage over thinner crusts and will generally overgrow them.

Figure 2. Crusts of equal thickness frequently reach a competitive stand-off.

Figure 3. Crusts with raised margins are able to overgrow thicker corallines with adherent margins of equal or greater thickness.



Figure 4. Corallines which regenerate secondary margins behind their primary margins, are able to slow down or stop the encroachment of thicker crusts.

In conclusion, two factors strongly influence a crust's competitive ability: the thallus thickness at the leading edge; and the height to which the growing margin is raised (Steneck, 1986; Steneck et al., 1991). In general, thicker corallines and those with raised margins have a competitive advantage over thinner ones and those with adherent margins of equal or greater thallus thickness (Paine, 1984; Steneck, 1986; Steneck et al., 1991). This suggests that thallus thickness and margin adherence are under strong selective pressure from interspecific competition.

It appears that this overgrowth model may also be applied to crustose and foliose lichens which also compete for space. The study by Keats & Maneveldt (1994) also shows that the regeneration of margins at the surface of the thallus by crusts like L. foveatum is able to reduce or stop the overgrowth by a thicker competitor and morphological features in addition to thallus thickness and raising of the margin can affect the outcome of competition for space.



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